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   难度：Easy
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   <h1 class="question_title">
    121. Best Time to Buy and Sell Stock
   </h1>
   <p>
    Say you have an array for which the
    <em>
     i
    </em>
    <sup>
     th
    </sup>
    element is the price of a given stock on day
    <em>
     i
    </em>
    .
   </p>
   <p>
    If you were only permitted to complete at most one transaction (i.e., buy one and sell one share of the stock), design an algorithm to find the maximum profit.
   </p>
   <p>
    Note that you cannot sell a stock before you buy one.
   </p>
   <p>
    <strong>
     Example 1:
    </strong>
   </p>
   <pre>
<strong>Input:</strong> [7,1,5,3,6,4]
<strong>Output:</strong> 5
<strong>Explanation:</strong> Buy on day 2 (price = 1) and sell on day 5 (price = 6), profit = 6-1 = 5.
&nbsp;            Not 7-1 = 6, as selling price needs to be larger than buying price.
</pre>
   <p>
    <strong>
     Example 2:
    </strong>
   </p>
   <pre>
<strong>Input:</strong> [7,6,4,3,1]
<strong>Output:</strong> 0
<strong>Explanation:</strong> In this case, no transaction is done, i.e. max profit = 0.
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   <h1 class="question_title">
    121. 买卖股票的最佳时机
   </h1>
   <p>
    给定一个数组，它的第&nbsp;
    <em>
     i
    </em>
    个元素是一支给定股票第
    <em>
     i
    </em>
    天的价格。
   </p>
   <p>
    如果你最多只允许完成一笔交易（即买入和卖出一支股票），设计一个算法来计算你所能获取的最大利润。
   </p>
   <p>
    注意你不能在买入股票前卖出股票。
   </p>
   <p>
    <strong>
     示例 1:
    </strong>
   </p>
   <pre><strong>输入:</strong> [7,1,5,3,6,4]
<strong>输出:</strong> 5
<strong>解释: </strong>在第 2 天（股票价格 = 1）的时候买入，在第 5 天（股票价格 = 6）的时候卖出，最大利润 = 6-1 = 5 。
     注意利润不能是 7-1 = 6, 因为卖出价格需要大于买入价格。
</pre>
   <p>
    <strong>
     示例 2:
    </strong>
   </p>
   <pre><strong>输入:</strong> [7,6,4,3,1]
<strong>输出:</strong> 0
<strong>解释: </strong>在这种情况下, 没有交易完成, 所以最大利润为 0。
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